As is known in the art, physical layer encryption (PLE) is a set of techniques that rely on information theory and the concept of channel capacity for security. Unlike traditional encryption, such as private- and public-key systems, PLE is not vulnerable to computational attacks and can offer perfect forward security. Many PLE techniques work by artificially degrading the eavesdropper's channel so that their channel capacity is not sufficient to recover the infonnation being sent. For example, a masking signal may be added to a communication signal such that it has a null in the direction of an intended receiver. For all other directions spatially separated from the intended receiver, an eavesdropper will receive a combination of the communication and masking signals, with the masking signal dominant. This degrades the information capacity of the eavesdropper channel, making it difficult or impossible to recover the transmitted information.
PLE is generally quantified by a measure called secrecy capacity. This represents the difference in channel capacity between the intended receiver and the eavesdropper. A positive secrecy capacity means that the intended receiver has a higher capacity than the eavesdropper and the communication link can be configured so that the receiver can demodulate the data and the eavesdropper cannot by choosing an appropriate rate and encoding scheme. If the secrecy capacity is negative, then the eavesdropper will be able to demodulate any message that the intended receiver can and secrecy fails.
One example a PLE technique is called Additive Artificial Noise (AAN) in which a transmitted signal is expressed as:xAAN(t)=w·s(t)+z(t)·n(t)  (1)z(t)∈N(h),∥z(t)∥=1  (2)where z(t) is a basis vector in the null space of the complex channel vector, vector h, s(t) is the communication signal, w is the set of complex beam-forming weights, and n(t) is a Gaussian random variable with variance selected according to the desired power division between signal and artificial noise. The choice of a basis vector in the null space of the channel ensures that the artificial noise does not appear in the intended receiver.
Another family of techniques is called Directional Modulation (DM), in which a different weighting vector is chosen for each symbol in the transmit constellation in order to form the desired vector at the intended receiver. This causes receivers in other positions to receive a constellation with distorted but still distinct symbols. Determining the necessary weighting vector is an unbounded problem and generally requires the use of matrix inversion or optimization techniques. An improvement on this technique chooses a different weighting vector each time a given symbol appears. This is sometimes called Dynamic DM. This addresses the vulnerability of so-called Static DM systems to eavesdropping techniques which can resolve the distorted constellation by changing the pattern of distortion continuously.